As is well known in the art, in electrophotographic developing methods, in general, a photoreceptor formed of a photoconductive material such as selenium, OPC (organic semiconductor), a-Si or the like has been used to form an electrostatic latent image thereon by various means. Then, by using a magnetic brush developing or the like, a toner charged with a polarity reverse to that of the latent image is attached thereonto by an electrostatic force to develop the latent image.
In the above developing step, there is used a two-component system developer comprising a toner and a carrier. The carrying particles called the carrier acts for imparting an appropriate positive or negative electric charge amount to the toner by frictional electrification, and delivering the toner into a developing zone near the surface of the photosensitive member on which the latent image is formed, by a developing sleeve in which magnets are accommodated, using a magnetic force thereof.
The electrophotography has been frequently used in extensive applications such as copying machines and printers. In recent years, there is an increasing demand for these equipments having a high image quality. In these application fields, with the increase in image quality, reduction in particle size of a developer used therein as well as increase in a copying or printing speed have proceeded. For this reason, a stress applied onto the developer tends to be increased, so that a large problem tends to be posed on difficulty in maintaining good characteristics of the developer.
With the market's requirements for personalization and space saving, etc., reduction in size of the electrophotographic image-forming apparatuses such as copying machines and printers has been promoted. with the reduction in size of these apparatuses, respective units thereof have also been required to have a reduced size. Thus, it has been required to provide a small-size developing device, i.e., maintain good developing characteristics of a developer used therein even when used in a small amount.
In general, in the apparatuses having a small size, in order to reduce an amount of an electric power consumed, there is a demand for a toner capable of sufficiently fixing images with a low fixing energy, i.e., a so-called low temperature fixable toner. The toner capable of ensuring a good fixing property at a low temperature by using a low-molecular weight resin, etc., can achieve saving of energy. However, owing to heat or pressure generated when repeatedly subjected to a plurality of developing operations for along period of time, occurrence of spent toner on a surface of the carrier tends to be caused by continuous use thereof under high-temperature and high-humidity conditions, or there tends to arise such a problem that carrier particles are strongly adhered to each other while entangling a toner between spent portions thereof, so that the developer tends to suffer from occurrence of blocking, resulting in variation in electrification amount of the developer and therefore variation in image density and occurrence of fogging, etc.
In view of these problems, in order to inhibit occurrence of spent toner on a surface of the carrier, there has been conventionally proposed the method in which the surface of the carrier is coated with various resins. For example, it is known that the surface of the respective core material particles of the carrier is coated with a releasable resin such as a fluororesin and a silicone resin. Such a coated carrier not only can be controlled with various functions such as an electric charge amount and an electric resistance, but also hardly occur spent toner on the surface of the magnetic carrier because the surface thereof is coated with the low-surface energy substance. As a result, the carrier has a stable electric charge amount, and the developer using the carrier exhibits a long service life. However, the fluororesin and the silicone resin exhibit weak adhesion to the core material of the carrier, so that there tends to occur such a durability problem that a coating layer of these resins is peeled off from the carrier when repeatedly used.
Hitherto, as the carrier constituting a two-component system developer, there are well known an iron powder carrier, a ferrite carrier, a carrier of a binder type in which magnetic particles are dispersed in a binder resin, and a carrier of a coated type in which a magnetic material is coated with a resin.
The iron powder carrier and ferrite carrier are usually used in the form of resin-coated particles. However, since the iron powder carrier has a true specific gravity as large as 7 to 8 g/cm3 and the ferrite carrier has a true specific gravity as large as 4.5 to 5.5 g/cm3, a large driving force is required for stirring these carriers in the developing device, resulting in significant mechanical damage to the device, occurrence of spent toner as well as deterioration in charging property of the carrier itself and accelerated damage to the photosensitive member. Further, since the adhesion between the surface of the particles and the coating resin is not good, the coating resin tends to be gradually peeled off during use with the time, thereby causing variation in the charging property. As a result, the problems such as formation of images defect and beads carry over tend to be caused.
The carriers of a magnetic material-dispersed type comprising spherical composite particles formed from magnetic particles and a phenol resin as described in Japanese Patent Application Laid-Open (KOKAI) No. 2-220068 and Japanese Patent Application Laid-Open (KOKAI) No. 2000-199985 have a true specific gravity as small as 3 to 4 g/cm3 as compared to the iron powder carrier or ferrite carrier, so that the energy upon impingement between the carrier and the toner becomes small. Therefore, it is advantageous to suppress occurrence of spent toner. Further, the carriers of the above type are far more excellent in adhesion to the coating resin, and therefore is almost free from such a problem that the coating resin is peeled-off during the use of the carrier.
With the recent progress of coloration of the toner, there is an increasing demand for carriers having a long service life for obtaining high-quality images. However, there tends to arise such a problem that the carriers are still insufficient to suppress occurrence of spent toner on a surface of the carrier particles, or peel-off or abrasion of the coating resin layer owing to impingement between the particles, mechanical agitation of the particles within a developing device and generation of heat caused thereby. Thus, it has been required that the carriers have a long service life capable of maintaining various properties such as a charging performance and an electric resistance over a long period of time.
Therefore, in order to further increase a service life of the magnetic carrier, it is strongly required to provide the magnetic carrier of a binder type which is capable of allowing a coating resin to strongly adhere onto the surface of the respective core material particles, is free from peeling or abrasion of the coating resin, and hardly suffers from occurrence of spent toner.
Conventionally, in order to enhance adhesion to a coating resin, there is described the magnetic carrier in which respective core material particles provided on a surface thereof with fine unevenness are coated with a resin (Patent Documents 1 to 3).